Laser welding method and monitoring method for secondary battery

ABSTRACT

Embodiments relate to laser welding methods, monitoring methods, and monitoring systems for a secondary battery. A laser welding method for a secondary battery includes performing laser welding on a positive electrode base having a thin-film shape in which a plurality of positive electrode base tabs are formed at a side, a negative electrode base having a thin-film shape in which a plurality of negative electrode base tabs are formed at a side, and a thin-film multi-tab to be joined to each of the positive electrode base and the negative electrode base, a welded portion in which the multi-tab is welded with the positive electrode base and the negative electrode base being melting-joined by using a laser such that a plurality of welding spots is formed on the welded portion.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2020-0019130, filed on Feb. 17, 2020 in the KoreanIntellectual Property Office, the entire disclosure of which isincorporated herein by reference.

BACKGROUND 1. Field

Aspects of embodiments of the present invention relate to laser weldingmethods, monitoring methods, and monitoring systems for a secondarybattery.

2. Description of the Related Art

A secondary battery includes an electrode assembly including a positiveelectrode, a negative electrode, and a separator disposed between thepositive electrode and the negative electrode and a cell including anelectrolyte impregnated to the electrode assembly.

In general, the electrode assembly of the secondary battery has astructure in which a negative electrode active material layer or apositive electrode active material layer is formed on a thin-film-typebase, and a negative electrode base tab or a positive electrode base tabis formed on an uncoated portion in which an active material layer isnot formed. Ultrasonic welding is used to bond a separate multi-tab to abase tab of the related art.

However, in case of the ultrasonic welding, only whether a weldedportion exists may be determined by an appearance inspection, and awelding defect, such as a weak welding state (a state having a lowbonding strength of a welded portion because welding is not properlyperformed), may not be determined. Also, since inspection for anultrasonic welded portion does not have a total quality verificationmethod except for the appearance inspection, reliability of theultrasonic welding may not be secured.

The above-described information described in this “Description of theRelated Art” is provided to improve understanding of the background ofthe embodiments, and, thus, may include information that is not part ofthe related art.

SUMMARY

According to aspects of embodiments of the present invention, laserwelding methods, monitoring methods, and monitoring systems for asecondary battery are capable of monitoring quality of a welded portionwhen laser welding is performed on a thin-film base and a multi-tab.According to further aspects of embodiments of the present invention,laser welding methods, monitoring methods, and monitoring systems for asecondary battery are provided which are capable of performing a totalinspection of a quality of a welded portion when laser welding isperformed on a thin-film base and a multi-tab.

According to one or more embodiments, a laser welding method for asecondary battery includes performing laser welding on a positiveelectrode base having a thin-film shape in which a plurality of positiveelectrode base tabs are formed at a side, a negative electrode basehaving a thin-film shape in which a plurality of negative electrode basetabs are formed at a side, and a thin-film multi-tab to be joined toeach of the positive electrode base and the negative electrode base, awelded portion in which the multi-tab is welded with the positiveelectrode base and the negative electrode base being melting-joined byusing a laser such that a plurality of welding spots is formed on thewelded portion.

A front bead protruding in an embossed shape from a front surface of thewelded portion and a back bead protruding in an embossed shape from arear surface of the welded portion may be formed in each of the weldingspots.

The back bead may have a diameter less than that of the front bead.

The multi-tab joined to the positive electrode base and the negativeelectrode base may be made of a different material than that of at leastone of the positive electrode base and the negative electrode base.

A welded portion of the negative electrode base may have a shearstrength of 4 kgf or less, and a welded portion of the positiveelectrode base may have a shear strength of 2 kgf or less. Here, theshear strength may allow all of the welded portions to be fractured.

According to one or more embodiments, a laser welding monitoring systemfor a secondary battery includes: a laser welding device configured toperform laser welding on a positive electrode base having a thin-filmshape in which a plurality of positive electrode base tabs are formed ata side, a negative electrode base having a thin-film shape in which aplurality of negative electrode base tabs are formed at a side, and athin-film multi-tab joined to each of the positive electrode base andthe negative electrode base, wherein a welded portion in which themulti-tab is welded with the positive electrode base and the negativeelectrode base is melting-joined by using a laser such that a pluralityof welding spots is formed on the welded portion; a perforated defectinspection device comprising a backlight configured to irradiate thewelded portion with light in a direction from a first surface toward asecond surface opposite the first surface of the welded portion, imageequipment arranged at an opposite side of the welded portion from thebacklight to collect light transmitted through the welded portion andform a first inspection image, and a controller configured to determinewhether a perforated defect of the welded portion exists by analyzingthe first inspection image; and a back bead inspection device comprisinga front light configured to irradiate the welded portion with light in adirection from the second surface toward the first surface of the weldedportion, image equipment arranged at a same side of the welded portionas the front light to collect light reflected by the welded portion andform a second inspection image, and a controller configured to determinewhether a back bead defect of the welded portion exists by analyzing thesecond inspection image.

The controller of the perforated defect inspection device may determinethat the welded portion is a welding defect when existence of athrough-hole is determined as light transmitted through the weldedportion is detected.

The controller of the back bead inspection device may determine that thewelded portion is a welding defect when existence of a through-hole isdetermined as light transmitted through the welded portion is detected.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present disclosure, and are incorporated in andconstitute a part of this specification. The drawings illustrate someexample embodiments of the present disclosure and, together with thedescription, serve to explain principles of the present disclosure. Inthe drawings:

FIG. 1 is a schematic block diagram illustrating a system constitutionfor monitoring a laser welding quality according to an embodiment;

FIG. 2A is a partial perspective view illustrating a welded portion whenlaser welding is performed according to an embodiment;

FIG. 2B is an enlarged plan view illustrating the welded portion of FIG.2A;

FIG. 3A shows photographs showing shapes of a front surface and a rearsurface of a positive electrode welded portion and a negative electrodewelded portion, on which the laser welding is applied, according to anembodiment;

FIG. 3B is a schematic diagram showing a front shape of a positiveelectrode welded portion and a negative electrode welded portion, onwhich laser welding is applied, according to an embodiment;

FIG. 3C is a schematic cross-sectional view illustrating a shape of thewelded portion according to an embodiment;

FIG. 4 is a schematic view illustrating a perforation inspection methodaccording to an embodiment;

FIG. 5A is a vision image illustrating a defective welded portion in aperforation inspection of FIG. 4;

FIG. 5B is an inspection image illustrating a defective welded portionin the perforation inspection of FIG. 4;

FIG. 6 is a view illustrating examples of defect determination in theperforation inspection of FIG. 4;

FIG. 7 is a schematic view illustrating a back bead inspection methodaccording to an embodiment;

FIG. 8A is a vision image illustrating a defective welded portion in aback bead inspection of FIG. 7;

FIG. 8B is an inspection image illustrating a defective welded portionin the back bead inspection of FIG. 7;

FIG. 9 is a view illustrating examples of defect determination of apositive electrode in the back bead inspection of FIG. 7; and

FIG. 10 is a view illustrating examples of defect determination of anegative electrode in the back bead inspection of FIG. 9.

DETAILED DESCRIPTION

The present invention will be described more fully herein with referenceto the accompanying drawings, in which some example embodiments of theinvention are shown. The example embodiments may, however, be embodiedin many different forms and should not be construed as being limited tothe embodiments set forth herein; rather, these embodiments are providedso that this disclosure will be thorough and complete, and will fullyconvey the scope of the present disclosure to those skilled in the art.

Also, in the figures, a thickness or dimension of each of layers may beexaggerated for clarity of illustration. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items. In this specification, it is to be understoodthat when a member A is referred to as being connected to a member B,the member A may be directly connected to the member B or indirectlyconnected to the member B with one or more members therebetween.

The terms used herein are for illustrative purposes of the presentdisclosure and should not be construed to limit the meaning or the scopeof the present disclosure. As used in this specification, a singularform may, unless definitely indicating a particular case in terms of thecontext, include a plural form. Also, the expressions “comprise” and/or“comprising” used in this specification neither define the mentionedshapes, numbers, steps, operations, members, elements, and/or groups ofthese, nor exclude the presence or addition of one or more otherdifferent shapes, numbers, steps, operations, members, elements, and/orgroups of these, or addition of these.

It is to be understood that, although the terms “first,” “second,” etc.may be used herein to describe various members, components, regions,layers, and/or sections, these members, components, regions, layers,and/or sections should not be limited by these terms. These terms areused to distinguish one member, component, region, layer, and/or sectionfrom another. Thus, a first member, a first component, a first region, afirst layer, and/or a first section discussed below could be termed asecond member, a second component, a second region, a second layer,and/or a second section without departing from the teachings of thepresent disclosure.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It is to be understood thatthe spatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “beneath” or “below” otherelements or features would then be oriented “above” or “over” the otherelements or features. Thus, the example term “beneath” can encompassboth an orientation of “above” and “below.”

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which the inventive concept pertains. Itis also to be understood that terms defined in commonly useddictionaries should be interpreted as having meanings consistent withthe meanings in the context of the related art, and are expresslydefined herein unless they are interpreted in an ideal or overly formalsense.

Herein, laser welding and a monitoring method for a secondary batteryaccording to some example embodiments will be described in furtherdetail with reference to the accompanying drawings.

FIG. 1 is a schematic block diagram illustrating a system constitutionfor monitoring a laser welding quality according to an embodiment.

As illustrated in FIG. 1, a laser welding device 100 is used to bond abase tab and a multi-tab of an electrode assembly of a secondarybattery. When laser welding is completed, whether a welded portion isdefective is automatically determined while the welded portionsequentially passes through a perforated defect inspection device 200and a back bead inspection device 300.

In an embodiment, the laser welding device 100 may be operated tospot-weld a welded portion by setting a pulse mode to minimize or reducea heat-affected zone when a thin-film-type welded portion is joined.

The perforated defect inspection device 200 automatically inspects adefect in which a welded portion is perforated instead of being joined.The perforated defect inspection device 200 may inspect whether a defectexists by fixing an object to be inspected (herein, referred to as aninspection target object) to a jig and irradiating, with a backlight210, at a rear side of the inspection target object. To this end,whether a defect of the laser welding exists is determined according towhether light passes by photographing whether light is transmitted to afront side of the inspection target object by using image equipment 220,such as a CCD camera. The determining of whether a defect exists may beautomatically performed by a controller 230 in the perforated defectinspection device 200.

The back bead inspection device 300 may inspect whether the defect ofthe laser welding exists by irradiating, with a front light 310, at afront side of the inspection target object, photographing lightreflected from a surface of the inspection target object by using imageequipment 320, such as the CCD camera, and inspecting whether a backbead is formed by using a shading difference (to this end, theinspection target object is disposed such that the back bead faces thefront light instead of the front bead). The determining of whether adefect exists may be automatically performed by a controller 330 in theback bead inspection device 300.

Herein, a welded portion of the secondary battery with theabove-described perforated defect inspection and back bead inspectionapplied will be described in further detail.

FIG. 2A is a partial perspective view illustrating a welded portion whenthe laser welding is performed according to an embodiment; and FIG. 2Bis an enlarged plan view illustrating the welded portion of FIG. 2A.FIG. 3A shows photographs showing shapes of a front surface and a rearsurface of a positive electrode welded portion and a negative electrodewelded portion, on which the laser welding is applied, according to anembodiment. FIG. 3B is a schematic diagram showing a front shape of apositive electrode welded portion and a negative electrode weldedportion, on which laser welding is applied, according to an embodiment.FIG. 3C is a schematic cross-sectional view illustrating a shape of thewelded portion according to an embodiment.

As illustrated in FIGS. 2A and 2B, in an embodiment, an electrodeassembly 400 has a structure including a pair of positive electrode basetabs 410 disposed in a positive electrode base and a pair of negativeelectrode base tabs 420 disposed in a negative electrode base. Each ofthe positive electrode base and the negative electrode base may have athin-film shape, and the base tab may also have a thin-film shape.

In an embodiment, each of the pair of positive electrode base tabs 410and the pair of negative electrode base tabs 420 are combined into onebody and then bent to a side, and a multi-tab 430 (or an electrode lead)is bonded to each of the bent positive electrode base tab 410 and thebent negative electrode base tab 420. The bonding method thereof may bethe above-described laser welding method, and a welded portion 500obtained by welding a portion of an area in which the base tab 410 and420 and the multi-tab 430 overlap each other may be formed. In anembodiment, a protection tape 440 may be attached to a connectionportion of a base with the positive electrode base tab 410 and thenegative electrode base tab 420, and a tab tape 450 may be attached tothe multi-tab 430 for insulation and protection.

In an embodiment, the laser welding may be performed concurrently (e.g.,simultaneously) on the welded portion of the positive electrode base tab410 and the negative electrode base tab 420 by the laser welding device100 of FIG. 1. In an embodiment, the laser welding is performed in apulse mode, and a welding bead is formed in a dot shape. That is, asillustrated in FIGS. 3A and 3B, a plurality of front beads 510 a and aplurality of back beads 510 b are formed on a front surface and a rearsurface of the welded portion 500 of the positive electrode base tab 410and the negative electrode base tab 420, respectively. Thus, when theback bead 510 b is not properly formed, or a hole is formed in thewelded portion 500 due to excessive melting, a welding defect may bedetermined.

While ultrasonic welding of the related art is a contacting andpressing-type welding method that is performed in a state of pressingthe welded portion, the laser welding is a melting-type bonding methodthat is performed in a non-contact manner and allows a material of thewelded portion to be melted. Thus, the laser welding has a weldingstrength greater than that of the ultrasonic welding.

In an embodiment, when a tensile force is measured with respect to acase that a force of pulling the welded portion of the positiveelectrode and the negative electrode, to which the laser weldingaccording to an embodiment is applied, the welded portion at thenegative electrode side may have a maximum shear strength of about 4kgf, and the welded portion at the positive electrode side may have amaximum shear strength of about 2 kgf. This result shows that a tensileforce of the negative electrode is greater by 1.5 kgf to 2 kgf and atensile force of the positive electrode is greater by 1.0 kgf to 1.5 kgfthan the ultrasonic welding.

Also, when standard deviations of the tensile forces of the weldedportion of the positive electrode and the negative electrode, to whichthe laser welding according to an embodiment is applied, and the weldedportion of the ultrasonic welding are compared, the standard deviationof the laser welding according to an embodiment is remarkably reduced tobe less by about 20% than that of the ultrasonic welding. That is, itmay be understood that the ultrasonic welding shows a wide qualitydistribution due to a great difference between the tensile forces of awelded portion on which the welding is properly performed and a weldedportion on which the welding is not properly performed and does notsecure reliability of the welded portion. However, it may be understoodthat when the laser welding according to an embodiment is applied, aquality distribution improves to be greater by 80% or more than that ofthe ultrasonic welding, and the reliability of the welded portion may besecured.

When the laser welding according to an embodiment is applied, as thewelding is performed to form two points or more of welding spots (pointsat which the front bead and the back bead are formed) at one weldedportion, shearing of the welded portion is not generated at all of a lowheight and a great height of the welded portion. That is, since thewelded portion in which two points to seven points of welding spots areformed maintains a welded state without shearing or fracturing even byperforming a drop test several tens of times under a general conditionand a high strength condition, the welding reliability may be improved.

The above-described welding spot according to an embodiment is shown inFIGS. 3A and 3C.

As illustrated in FIGS. 3A and 3C, the front bead 510 a formed on afront surface 402 of the welded portion at the positive electrode sidehas a circular shape on a plane, and protrudes to have asemi-circular/semi-elliptical shape on a side surface or a cross-sectionas in FIG. 3C. In comparison, the back bead 510 b formed on a rearsurface 404 of the welded portion at the positive electrode side mayhave a diameter that is slightly smaller than that of the front bead 510a and a protruding height that is also less than that of the front bead510 a.

The front bead 510 a formed on a front surface 406 of the welded portionat the negative electrode side may have a diameter that is equal to orless than that of the front bead 510 a at the positive electrode sideand a protruding height that is also equal to or less than that of thefront bead 510 a at the positive electrode side. This is becausematerials of the positive electrode base tab, the negative electrodebase tab, and the multi-tab have different reflectance and absorptancewith respect to a laser beam. Also, the back bead 510 b formed on a rearsurface 408 of the welded portion at the negative electrode side mayhave a diameter and a protruding height less than those of the frontbead 510 a formed on the front surface 406 of the welded portion at thenegative electrode side.

In an embodiment, for example, the front bead 510 a of the weldedportion at the positive electrode side and the negative electrode sidemay protrude in an embossed shape having a diameter of about 0.3 mm toabout 0.4 mm, and a distance between central points of the welding spotsmay be about 0.6 mm. In an embodiment, the back bead 510 b of the weldedportion at the positive electrode side and the negative electrode sidemay have a diameter that is about 30% to about 70% of the diameter ofthe front bead 510 a. However, the above-described numerical values maybe changed through setting variation of the laser welding device.

Hereinabove, an embodiment in which each of the front bead and the backbead at the positive electrode side and the negative electrode side hasa circular shape is described. However, embodiments are not limited tothe circular shape of each of the front bead and the back bead. Forexample, as illustrated in FIG. 3B, the front bead may have a shape of aplurality of straight lines or a continuous spiral shape. Also, thefront bead may have a shape of a symbol or a specific character such as‘C’ or ‘0’. That is, the shape of the front bead may be variouslychanged. The shape of the back bead may also be variously changedaccording to the shape of the front bead.

In an embodiment, in the positive electrode, all of the positiveelectrode base tab 410 and the multi-tab 430 are mainly made of aluminumthat is a material having a high reflectance (e.g., a reflectance of90%). In an embodiment, in the negative electrode, the negativeelectrode base tab 420 is mainly made of copper that is a materialhaving a high reflectance (e.g., a reflectance of 93%), and themulti-tab 430 is mainly made of nickel that is a material having a lowreflectance (e.g., a reflectance of 72%). Thus, in an embodiment, in thepositive electrode, welding is performed between the same kind ofmaterials having the same melting point (e.g., about 660.2° C.), and, inthe negative electrode, welding is performed between different kinds ofmaterials having different melting points (e.g., about 1083±1° C. forcopper and about 1455±1° C. for nickel). Therefore, the negativeelectrode has a disadvantage in welding condition in comparison with thepositive electrode. Thus, a wavelength of the laser is set inconsideration of a melting point, a reflectance, and a laser absorptanceof the tab material.

For example, in an embodiment, in the negative electrode, as the nickelmulti-tab 430 is disposed at an upper portion, and the copper base tab420 is disposed at a lower portion, the welding may be performed basedon a melting point of nickel because the nickel is disposed at the upperportion although the copper has a low melting point. Also, in thepositive electrode, since the welding is performed between the same kindof materials and between materials having the high reflectance, awelding time may be equal to that in case of the negative electrode, buta laser output for the welding of the positive electrode may be greaterthan that for the welding of the negative electrode. In consideration ofthe above-described features, in an embodiment, the welding may beperformed by setting a laser wavelength of 1070 nm that is an infraredregion, and the laser output for welding of the positive electrode maybe set to be greater by 2% than that for welding of the negativeelectrode based on a laser output of 300 W.

Also, in an embodiment, as spot welding is performed by using a pulsemode when irradiated with the laser, a heat-affected zone generated dueto heat accumulation during welding may be minimized or reduced.

However, a welding defect may be generated despite the above-describedeffort, and an embodiment discloses a method for automaticallyinspecting whether the welding defect is generated to resolveinconvenience of the related art that inspects by using only the nakedeyes of a worker.

Herein, a method for determining whether a welding defect of the weldedportion is generated will be described in further detail.

FIG. 4 is a schematic view illustrating a perforation inspection methodaccording to an embodiment. FIG. 5A is a vision image illustrating adefective welded portion in the perforation inspection of FIG. 4. FIG.5B is an inspection image illustrating a defective welded portion in theperforation inspection of FIG. 4. FIG. 6 is a view illustrating examplesof defect determination in the perforation inspection of FIG. 4.

As illustrated in FIG. 4, a pair of jigs 240 is mounted to the electrodeassembly 400, and the welded portion is irradiated with the backlight210 disposed at a rear surface of the welded portion. Thereafter,whether light is transmitted through the welded portion is inspected byusing the image equipment 220 disposed in a front direction of theelectrode assembly 400. When the welded portion of the electrodeassembly 400 is perforated due to over-welding, the light irradiatedfrom the backlight 210 may be leaked to the front surface, and the imageequipment 220 may detect the leaked light.

FIG. 5A is an image (a vision image) obtained by light transmittedthrough a through-hole when viewed by the naked eyes, and FIG. 5B is aninspection image photographed by the image equipment 220. Since theimage viewed by the naked eyes and the inspection image photographed bythe image equipment 220 show the same shape as each other, whether theperforated defect exists may be determined by the image photographed bythe image equipment 220.

As illustrated in FIG. 6, since the vision image and the inspectionimage are the same as each other although all of the numbers andpositions of perforations are different, the defect determination ispossible. Since the welded portion is not perforated when the back beadhaving a normal shape is formed on the welded portion, all ofthrough-holes are determined as defects regardless of the numbers andpositions thereof.

When the perforation inspection is completely performed on the electrodeassembly 400, the electrode assembly 400 moves, and a back beadinspection is performed.

FIG. 7 is a schematic view simply illustrating the back bead inspectionmethod according to an embodiment. FIG. 8A is a vision imageillustrating a defective welded portion in the back bead inspection ofFIG. 7. FIG. 8B is an inspection image illustrating a defective weldedportion in the back bead inspection of FIG. 7. FIG. 9 is a viewillustrating examples of defect determination of the positive electrodein the back bead inspection of FIG. 7. FIG. 10 is a view illustratingexamples of defect determination of the negative electrode in theperforation inspection of FIG. 9.

As illustrated in FIG. 7, the electrode assembly 400 is mounted to apair of jigs 340 (or the jigs 240 used in the previous process), and thewelded portion is irradiated with the front light 310 disposed in frontof the welded portion. Thereafter, as reflected light is collected bythe image equipment 320 disposed in a front direction of the electrodeassembly 400, an image is acquired. Whether a defect of the weldedportion exists is determined by generating an inspection image based onshading difference in the acquired image and then sensing whether theshape of the back bead exists. Whether the number and position of theback bead are defective may be determined in comparison with a settingimage that is preset in the controller 330.

FIG. 8A is an image (a vision image) of the welded portion on which theback bead is formed when viewed by the naked eyes, and FIG. 8B is aninspection image photographed by the image equipment 320. Since theimage viewed by the naked eyes and the inspection image photographed bythe image equipment 320 show the same shape as each other, whether theback bead defect exists may be determined by the image photographed bythe image equipment 320. Also, since only three back beads out of fourback beads are formed, a finial defect determination is determined as adefect (NG) (refer to determination on a shape of fifth back bead inFIG. 10).

As illustrated in FIGS. 9 and 10, since the vision image and theinspection image are the same as each other although all of the numbersand positions of perforations are different, the defect determination ispossible. Since the number and shape of the back bead are acquired assame as the setting image when the back bead having the normal shape isformed on the welded portion, when the preset number of back beads arenot detected, all of the welded portions are determined as defects.

As illustrated in FIGS. 9 and 10, although the shape of the back bead ofthe welded portion at the positive electrode base tab side and the shapeof the back bead of the welded portion at the negative electrode basetab side are slightly different, the number and the position may bechecked on the vision image. Thus, when the number of the back bead inthe inspection image is the same as the preset number although shapesand sizes are different, the controller 330 determines that the weldingis properly performed (refer to determination OK in FIGS. 9 and 10).When the number of the back bead detected from the inspection image isdifferent from the preset number, the controller 330 determines that thewelding is defective (refer to determination NG in FIGS. 9 and 10).

As described above, according to an embodiment, whether the weldedportion 500 of the electrode assembly 400 is defective may beautomatically determined through the perforated defect inspection device200 and the back bead inspection device 300 instead of being determinedby the naked eyes of the worker.

Thus, a welding defect such as the perforation in the welded portion andthe weak welding, such as the back bead that is not properly formed, maybe inspected, the working time may be reduced, and the workingefficiency may be improved.

According to an embodiment, whether the perforated defect exists may beautomatically inspected by irradiating the rear surface of the object tobe welded with the local lighting and detecting the light transmittedthrough a perforated portion.

Also, according to an embodiment, since the light reflected from thewelding back bead may be detected by irradiating the front surface ofthe object to be welded with the light, and the welding shape image maybe automatically generated by the shading difference, a welding defectand weak welding of the welded portion may be inspected.

The above-described embodiments are provided as example embodiments,and, thus, the present invention is not limited to the foregoingembodiments, and also it will be understood by those of ordinary skillin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present invention asset forth in the following claims.

What is claimed is:
 1. A laser welding method for a secondary battery,the method comprising: performing laser welding on a positive electrodebase having a thin-film shape in which a plurality of positive electrodebase tabs are formed at a side, a negative electrode base having athin-film shape in which a plurality of negative electrode base tabs areformed at a side, and a thin-film multi-tab to be joined to each of thepositive electrode base and the negative electrode base, wherein awelded portion in which the multi-tab is welded with the positiveelectrode base and the negative electrode base is melting-joined byusing a laser such that a plurality of welding spots is formed on thewelded portion.
 2. The laser welding method of claim 1, wherein a frontbead protruding in an embossed shape from a front surface of the weldedportion and a back bead protruding in an embossed shape from a rearsurface of the welded portion are formed in each of the welding spots.3. The laser welding method of claim 2, wherein the back bead has adiameter less than that of the front bead.
 4. The laser welding methodof claim 1, wherein the multi-tab joined to the positive electrode baseand the negative electrode base is made of a different material thanthat of at least one of the positive electrode base and the negativeelectrode base.
 5. The laser welding method of claim 4, wherein a weldedportion of the negative electrode base has a shear strength of 4 kgf orless, and a welded portion of the positive electrode base has a shearstrength of 2 kgf or less, and wherein the shear strength allows all ofthe welded portions to be fractured.
 6. A laser welding monitoringsystem for a secondary battery, the system comprising: a laser weldingdevice configured to perform laser welding on a positive electrode basehaving a thin-film shape in which a plurality of positive electrode basetabs are formed at a side, a negative electrode base having a thin-filmshape in which a plurality of negative electrode base tabs are formed ata side, and a thin-film multi-tab to be joined to each of the positiveelectrode base and the negative electrode base, wherein a welded portionin which the multi-tab is welded with the positive electrode base andthe negative electrode base is melting-joined by using a laser such thata plurality of welding spots is formed on the welded portion; aperforated defect inspection device comprising a backlight configured toirradiate the welded portion with light in a direction from a firstsurface toward a second surface opposite the first surface of the weldedportion, image equipment arranged at an opposite side of the weldedportion from the backlight to collect light transmitted through thewelded portion and form a first inspection image, and a controllerconfigured to determine whether a perforated defect of the weldedportion exists by analyzing the first inspection image; and a back beadinspection device comprising a front light configured to irradiate thewelded portion with light in a direction from the second surface towardthe first surface of the welded portion, image equipment arranged at asame side of the welded portion as the front light to collect lightreflected by the welded portion and form a second inspection image, anda controller configured to determine whether a back bead defect of thewelded portion exists by analyzing the second inspection image.
 7. Thelaser welding monitoring system of claim 6, wherein the controller ofthe perforated defect inspection device determines that the weldedportion is a welding defect when existence of a through-hole isdetermined as light transmitted through the welded portion is detected.8. The laser welding monitoring system of claim 6, wherein thecontroller of the back bead inspection device determines that the weldedportion is a welding defect when existence of a through-hole isdetermined as light transmitted through the welded portion is detected.